1. Field of the Invention
The present invention relates to stepping motors used as actuators of various control devices, and in particular, a stepping motor suitable for smoothly driving an indicator of a speed meter or the like.
2. Description of the Related Art
Stepping motors are widely used in various industrial fields. Particularly, they have been increasingly applied to rotational driving devices which require delicate movement.
In FIG. 6, numerals 31a and 31b represent cylindrical bobbins and coils 32 are wound on their peripheries.
These bobbins 31a and 31b, on which coils 32 are wound, are clamped by circular outer yokes 33a and 33b and circular inner yokes 34a and 34b.
On inner peripheries of the outer yokes 33a and 33b and the inner yokes 34a and 34b, pole teeth 36a, 36b and 36c and 36d are protrusively provided, and these outer yokes 33a and 33b and these inner yokes 34a and 34b are placed parallel with a predetermined shift along the periphery so that the pole teeth 36a and 36b and the pole teeth 36c and 36d engage each other at a central space section 37.
The pole teeth 36a and 36b and the pole teeth 36c and 36d form opposite magnetic poles, respectively, when the coils 32 are energized.
The first yokes 33a and 34a and the second yokes 33b and 34b are stacked up through a non-permeable spacer (not shown in the drawing) so that the inner yokes 34a and 34b face each other with a given space there between, and a rotor 39 having a shaft 38 to transmit a rotational driving force is inserted in the central space section 37 of the first yokes 33a and 34a and the second yokes 33b and 34b.
Multiple alternative polarization is performed on the outer periphery of the rotor 39, and the rotor 39 rotates by means of magnetic attractive force between the pole teeth 36a, 36b, 36c and 36d.
The stepping motor is assembled by rotatably supporting an end of the shaft 38 of the rotor 39 on a bearing stand 40a in the center of a plate 40, by covering a permeable cup frame 41 on the other end of the shaft 38, and rotatably supporting the other end of the shaft 38 with a bearing stand 41a provided in the cup frame 41.
The bobbins 31a and 31b have pole teeth inside their respective central cavities. In the stepping motor set forth in FIG. 6, the pole teeth of the bobbin 31a and the pole teeth of the bobbin 31b must be shifted at a given angle to each other when assembling them.
In such a configuration, however, magnetic balance of the bobbins 31a and 31b cannot be readily achieved during assembly, resulting in stepping angle errors of the shaft 38.
Since the cup frame 41 is formed by press working, its production cost is high and the cap tends to have a tapered shape. Therefore, the inside face of the cylindrical section comes in insufficient contact with the yokes 31a and 31b to achieve a precise torque generation and stepping angle. Further, an additional assembly step, such as cramping or welding, is required for contact of the cup frame with the yokes.
In order to rotate the rotor 39, a current flow is conducted to the coils 32 to energize the outer yokes 33a and 33b and the inner yokes 34a and 34b and to generate magnetic attractive force between the pole teeth 36a, 36b, 36c and 36d. Thus, the outer yoke 33a and the inner yoke 34a, and the outer yoke 33b and the inner yoke 34b must be magnetically coupled to form respective magnetic circuits.
The magnetic coupling has been conventionally achieved by a permeable cup frame 41. In this case, since the magnetic coupling is simultaneously performed between the first yokes 33a and 34b and second yokes 33b and 34b, a magnetic circuit of the first yokes 33a and 34a and a magnetic circuit of the second yokes 33b and 34b will lead each other through the cup frame to generate magnetic interference depending on the magnetizing patterns of the coils 32.
In detail, as set forth in FIG. 7(a), when the pole teeth 36b and 36c have the same polarity, they repel each other and thus no magnetic interference occurs. When the pole teeth 36b and 36c have opposite polarities due to change in the magnetizing pattern as set forth in FIG. 7(b), a magnetic field from the pole teeth 36b to the pole teeth 36c through the cup frame 41 is generated and magnetic interference occurs between the first and second yokes.
Since the magnetic interference depends on the change in magnetizing pattern, irregular rotation occurs due to the change.
When using this stepping motor for driving an indicator of a speed meter, the indicator does not move smoothly.
An idea for solving such problems involves two upper and lower cup frames 41 with each cup frame independently covering the outer peripheries of the first yokes 33a and 34a and the second yokes 33b and 34b. In this case, two expensive cup frames must be prepared. The improvement of the present invention is directed to overcoming such problems.